Effect Of Co Additive On The Microstructures And Hardenability Of Aluminium Alloy7085

Aluminum alloy7085is a patented alloy developed by Alcoa company in the United States in2002,which is on the basis of aluminum alloys7050and7150by lowering their impurities of Si and Mn contents and improving Zn/Mg ratio.It has high strength,good toughness,fatigue resistance and low quenching sensitivity and the hardenable thickness can be up to300mm.But with the requirements of lowering the weight and improving the efficiency of aerospace equipments improve constantly,the demands of large and thick aluminum materials are more and more urgent.The hardenability of aluminum alloy7085needs further improvement.This article adds0,0.05,0.1,0.2,0.4%Co to the matrix alloy-the aluminum alloy7085(mass fraction wt%:7.0-8.0Zn,1.2～1.8Mg,1.3-2.0Cu,0.08-0.15Zr) through casting metallurgical method,and the melting result of Co contents in the alloys is0,0.05,0.095,0.2,0.39%.Using optical microscopy,scanning electron microscopy and transmission electron microscopy,the microstructures of alloys and DSC curves are observed. The hardness,electrical conductivity,tensile properties,corrosion resistance after different cooling ways (room temperature water cooling, air cooling) and aging (artificial aging and natural aging) are tested and the effect of Co additive on the hardenability is analized.The main conclusions are as following:(1) The hardenability increases from92%of the matrix aluminum alloy7085to95%of0.095%Co content alloy during end-quenching length range of100mm,and the hardenability increases by3%.While too little (0.05%) or too much (≥0.2%) Co content will lower the hardenability of matrix alloy.(2) Adding an appropriate amount of Co(0.095%) will reduce the strength quenching sensitivity of this alloy,but adding excessive Co(0.39%) will increase the strength quenching sensitivity of the alloy. When adding0.39%Co,the hardness and mechanical tensile strength of alloy after water quenching will be higher than matrix aluminum alloy7085,which is associated with the formation of a large number of high hardness Co-rich phases and these phases can produce multiphase strengthening.While the hardnesses of other alloys are lower than the matrix alloy.(3) Adding different Co may affect the quantity of the particles triggering recrystallization nucleation,which affects the degree of recrystallization and makes differences of the coarse rj equilibrium phase particles’number and distribution after quenching and aging.That is the direct result of the quenching sensitivity differences.The degree of recrystallization is2#(0.05%Co>5#(0.39%Co)>4#(0.2%Co)>l#(0%Co)>3(0.095%Co),and the recrystallization of3alloy was significantly lower than the other four kinds of alloys.This suggests that adding0.095%Co can improve the ability of alloy to inhibit recrystallization,which is advantageous to keep the deformation organization. And this can decrease the precipitation of coarse η equilibrium phases during slow quenching,which can improve the hardenability of this alloy.(4) With the increase of Co content,the conductivities of alloys continue to decline.For natural aging alloys,with the extension of natural aging time,the conductivities of alloys after two cooling ways will continue to reduce.For the artificial aging alloys,with the extension of artificial aging,the conductivities of alloys after air cooling will continue to increase,while the conductivities of alloys after water quenching will go through rapid decline and then continue to rise.(5) The corrosion resistance of water-cooled alloys are superior to the air cooling alloys;to the alloys of the same heat treatment state, with the increase of Co content,too little (0.05%) or too much (≥0.2%) Co content will lower the corrosion resistance of alloys,while0.095%Co content will improve the corrosion resistance of alloys.